This invention relates to a mass spectrometer radio frequency (RF) power supply for applying a RF field to an ion storage device and to a method of operating an ion storage device using a RF field. In particular, but not exclusively, this invention relates to an ion storage device that contains or traps ions using a RF field prior to ejection to a pulsed mass analyser.
Such traps could be used in order to provide a buffer for the incoming stream of ions and to prepare a packet with spatial, angular and temporal characteristics adequate for the specific mass analyser. Examples of pulsed mass analysers include time-of-flight (TOF), Fourier transform ion cyclotron resonance (FT ICR), Orbitrap types (i.e. those using electrostatic only trapping), or a further ion trap. A block diagram of a typical mass spectrometer with an ion trap is shown in FIG. 1. The mass spectrometer comprises an ion source that generates and supplies ions to be analysed to an ion trap where the ions are collected until a desired quantity are available for subsequent analysis. A first detector may be located adjacent to the ion trap so that mass spectra may be taken, under the direction of the controller. The pulsed mass analyser is also operated under the direction of the controller. The mass spectrometer is generally provided within a vacuum chamber provided with one or more pumps to evacuate its interior.
Ion storage devices that use RF fields for transporting or storing ions have become standard in mass spectrometers, such as the one shown in FIG. 1. Typically, they include a RF signal generator that provides a RF signal to the primary winding of a transformer. A secondary winding of the transformer is connected to the electrodes (typically four) of the storage device. FIG. 2a shows a typical arrangement of four electrodes in a linear ion trap device. The elongate electrodes extend along a z axis, the electrodes being paired in the x and y axes. The electrodes are shaped to create a quadrupolar RF field with hyperbolic equi-potentials that contain ions entering or created in the trapping device. Trapping within the storage device is assisted by the use of a DC field. As can be seen from FIG. 2a, each of the four elongate electrodes is split into three along the z axis. Elevated DC potentials are applied to the front and back sections of each electrode relative to the larger central section, thereby superimposing a potential well on the trapping field of the ion storage device that results from the superposition of RF and DC field components. AC potentials may also be applied to the electrodes to create an AC field component that assists in ion selection.
FIGS. 2b and 2c show typical potentials applied to the electrodes. Of most interest is FIG. 2c that shows the RF potentials which concern this invention. As can be seen, like potentials are applied to opposed electrodes such that the x-axis electrodes have a potential of opposite polarity to that of the y-axis electrodes.
FIG. 3 shows a power supply capable of providing the desired RF potentials. A RF generator supplies a RF signal to a primary winding of a transformer, as mentioned above. This signal is coupled to the secondary winding of the transformer. One end of the secondary winding is connected to the x-axis pair of opposed electrodes, the other end is connected to the other, y-axis pair of opposed electrodes. A DC offset may be applied using a DC supply connected to a central tap of the secondary winding. AC potentials can also be applied to the electrodes, but this aspect of the storage device need not be considered here.
Further details of this type of ion storage device can be found in U.S. Patent Application Publication No. 2003/0173524.
The inductance in the coils comprising the winding of the transformer and the capacitance between the electrodes forms an LC circuit. The transformer corresponds to high quality resonance coils, with a quality factor reaching many tens or even hundreds. This produces RF amplitudes up to thousands of Volts at working frequencies normally in the range of 0.5-6 MHz.
Such storage devices are often used to store ions prior to ejection to a subsequent mass analyser. Whenever such storage devices are interfaced to other analysers, especially pulsed ones (e.g. to a TOF mass analyser or an electrostatic-only trapping mass analyser such as the Orbitrap mass analyser), a problem of efficient transfer of ions from the storage device to the analyser becomes a stumbling block. When 3D quadrupole RF traps are used as storage devices as the first stage of mass analysis, this problem is traditionally solved by pulsing DC potentials on end-cups of the ion trap in synchronisation with switching off the RF signal generator (S. M. Michael, M. Chien, D. M. Lubman, Rev. Sci. Instrum. 63(10) (1992) 4277-4284). This normally allows extraction of ions from the ion trap, the extraction being facilitated by the typically favourable aspect ratio (i.e. length/width) of the 3D trap. However, the same factor is also responsible for a limited storage volume and hence limited space charge capacity of the 3D trap. Due to the relatively slow and voltage-dependent switching off transition of RF signal generators, resolving power (and, presumably, mass accuracy) of the storage device is severely compromised.
The linear ion trap provides orders of magnitude greater space charge capacity, but its aspect ratio makes direct coupling to pulsed analysers very difficult. Usually, this is caused by the vast incompatability of time scales of ion extraction from the RF storage device (ms) and peak width required for pulsed analysers (ns). This incomparability can be reduced by compressing ions along the axis and then ejecting ions out axially with high-voltage pulses (WO02/078046). However, space charge effects become very important in this case.
The above devices use axial ejection, but an alternative is to eject ions orthogonal to the axis of the storage device (see, for example, U.S. Pat. No. 5,420,425, U.S. Pat. No. 5,763,878, US2002/0092980 and WO02/078046). For this, DC voltages on opposing rod electrodes are biased in such a way that ions are accelerated through one electrode into the subsequent mass analyser. It is also disclosed that the RF potential on electrodes of the storage device should be switched off in order to limit energy spread and mass-dependence of ion energy. However, these disclosures only state the objective of switching off the RF field at zero phases and do not describe how this could be done. All of the above disclosures (except WO02/078046) relate only to ion storage devices using straight electrodes and only in application to TOFMS.
WO00/38312 and WO00/175935 describe switching off RF potentials on the electrodes of a storage device in a 3D trap/TOFMS hybrid mass spectrometer. These documents disclose switching resonance coils but this has the disadvantage of requiring power supplies with opposite polarities, as well as two high-voltage pulsers for each RF voltage. Large discharge currents impose excessive loads on these power supplies that can be only partly alleviated by adding capacitance in parallel. Also, internal capacitance of pulsers adds to that of the coil thus reducing its resonant frequency. These disclosures do not show how to switch RF off on more than one electrode or on multi-filar coils, or how to combine RF switching with pulsed DC offsets of electrodes of the RF device. The optimum use of this scheme is the rapid start of RF voltage rather than rapid switch-off. Unfortunately, ejection of ions into the subsequent mass analyser requires high speed of switch-off, while switch-on could be considerably slower for typically used quasi-continuous ion sources.
WO00/249067 and US2002/0162957 disclose switching RF off for a 3D trap mass spectrometer (a leak detector) in order to achieve ion ejection without the use of any DC pulses. However, these documents do not disclose any viable schemes of RF switching except conventional powering down of the primary winding of the coil or use of slow mechanical relays.
Another example of RF switching for a cylindrical trap/TOFMS hybrid has been disclosed by M. Davenport et al, in Proc. ASMS Conf., Portland, 1996, p. 790, and by Q. Ji, M. Davenport, C. Enke, J. Holland, in J. American Soc. Mass Spectrom, 7, 1996, 1009-1017. This scheme utilises two fast break-before-make switches each consisting of two pairs of MOSFETs (per each phase of RF). The circuit's rating is limited by the rating of the MOSFETs (900 V), and the quality of the RF circuit is severely limited by the high capacitance of the MOSFETs (ca. 100 pF each) that is also aggravated by the large number of these elements.